Studies of a Liquid Argon Time Projection Chamber in a Magnetic Field and at High Pressure A Proposal to the DOE Advanced Detector Research Program

Recent results from SNO [1] and KamLAND [2] confirm the large mixing angle MSW solution to the solar neutrino problem, which implies that the full 3-generation neutrino mixing matrix can be studied in a new generation of accelerator-based neutrino experiments. A particular opportunity for this is to place a new, large (20 kton or more) neutrino detector in a NuMI off-axis beam [3], where parameters sin 2θ13, the sign of ∆M 2 23, and the CPviolating phase δ could be studied. At this scale of detector, the best technology – a total absorption tracking calorimeter based on a liquid argon time-projection chamber (TPC) [4, 5, 6, 7] – is very cost competitive with coarser sampling devices. Furthermore, a small (40 ton) liquid argon TPC is well suited for use as a near detector in the NuMI beam to characterize the beam, measure low-energy charged and neutral current neutrino interaction cross sections, and to perform nonoscillation physics measurements that emphasize aspects of nuclear structure (such as the strange-quark sea) best accessible via neutrino beams. A liquid argon TPC is compatible with operation inside a magnetic field, which would permit measurement of final-state muon momenta in charged-current interactions, and identification of the sign of final-state electrons/positrons. The former capability is desirable in a near detector application, and both capabilities are useful, even essential in a detector for a neutrino factory based on a muon storage ring [8]. The technology of liquid argon TPC’s is now relatively mature due to the extensive efforts of the ICARUS group [9], whose largest device to date has a mass of 600 tons. In extrapolating liquid argon TPC’s to future applications in neutrino beams [10], several issues will benefit from R&D: operation with extremely long wires, operation at high pressure, operation in a magnetic field, upgrades of electronics and cryogenic feedthroughs, industrialscale purification of argon to 0.1 ppb of O2. Here, we propose to undertake R&D on the two of these topics that can be addressed by a relatively small-scale effort: 1. Verification that a liquid argon detector can be operated with the electric field perpendicular to the magnetic field (unlike gas phase time projection chambers that must be operated with E parallel to B). 2. Verification that a liquid argon detector can be operated at several atmospheres pressure, as would occur near the bottom of a very large detector. Corresponding author: [email protected]